5.3L LS Small Block Build - Fuel Injection, Part 6

We return to fuel injection on our stroked 5.3L test mule Modern Mouse.

How is the runner length tuned to provide additional airflow? In addition to providing a simple airflow path to the combustion chamber, the intake runner (combined with the head port) provides additional cylinder filling by way of both an inertial and reflected wave ram tuning. The inertial wave cylinder filling happens when the downward movement of the pistons (and open intake valve) creates a vacuum, which initiates movement of the air column. If timed properly (determined by the length and inside diameter of the inlet tract), the inertia of the air column can improve cylinder filling beyond the vacuum created by the downward moving of the piston.

The reflected wave provides additional cylinder filing as well, but by a different means. The reflected wave happens when the intake valve opens to create a negative pressure wave. This negative pressure wave travels out away from the valve until finally arriving at the common plenum. The negative pressure wave expands out into the open plenum. The expansion of the negative pressure wave creates a low-pressure area that is quickly filled. The filling of the void creates a positive pressure wave, which then travels back down the intake port and into the combustion chamber, improving cylinder filling.

While both inertia and reflected wave ram tuning improve cylinder filling, the key is timing. The pressure waves must arrive at the proper time to maximize cylinder filling. Obviously they must also not overlap or work against each other (where a positive pressure wave is effectively cancelled out by a negative one). The runner length (and diameter) help determine when (in the rpm range) these forms of cylinder filling become effective. Since the pressure waves run through the intake runners (and head port) at the speed of sound, altering the distance changes the travel time through the port. The travel time can also be altered (to a lesser extent) with changes in temperature and pressure. If the runner length is properly tuned, the motor will realize not only the airflow drawn in by way of the vacuum created by the downward moving piston, but also the mild supercharging effect that comes from the inertial and reflected wave tuning.

A third form of wave tuning is called Helmholtz resonance. This deals with the resonance of the common plenum and tuned length of inlet tubing (from the air filter to the intake manifold). Once excited, the common plenum resonates at a given frequency. These resonance waves can be used to bombard the intake valve with additional pressure, once again assuming the tuned frequency coincides with the effective operating speed of the motor. Note that these forms of ram tuning are irrespective of the airflow potential of the port and function even in boosted applications.

While we have simply scratched the surface of the intake manifold, it should be clear that the proper design is much more complicated than simply building runners connected to a common plenum fed by a throttle body. Obviously computer simulations can be used to design an intake manifold for a given application, but given the complex nature and sheer number of pressure waves involved (most intake modeling is done with a single-cylinder motor), the final testing almost always involves actual dyno testing. Though it is possible to determine (or design) the effective operating range of the intake manifold runner length, diameter, and plenum, what effect does the design have at other engine speeds? Is the area under the curve sufficient? Are there any undesirable dips or peak? These are questions that only the dyno can answer effectively. Thus a proper intake design should see plenty of dyno time before being finalized. Obviously it would be nice if the same intake fit properly, bolted up to all the original hardware and looking like a proper performance piece.

MORE PHOTOS

Having run dual quads back on Modern Mouse in part 5, we jumped back to the future to test the BBK SSI EFI intake.

1 Unlike the factory truck, LS1 or LS6 intakes, the BBK SSI featured aluminum construction. Though ours came equipped with a titanium finish, it is possible to polish the aluminum.

2 The BBK SSI intake features a throttle opening designed to accept an 80mm or 85mm BBK throttle body.

3 The SSI intake featured a dedicated port designed to accept the factory MAP sensor.

4 We like the fact that the SSI intake design featured a removable lower cover. The cover provided access to port the intake runners for improved flow and power. We left the intake as-cast for our testing.

6 Loyal readers will remember that our 5.3L test motor is now sporting a 383 stroker kit courtesy of Procomp Electronics and Probe Racing. L&R Automotive was responsible for machine work and balancing of the stroker assembly.

7 Feeding Modern Mouse was a set of Gen X 215 heads from Trick Flow Specialties. The heads featured full CNC porting and were perfectly sized for our 383 LS.

8 The stroker had received its fair share of cam profiles in the past but was currently sporting a dual-pattern, Comp hydraulic roller profile. The LSR cathedral-port grind offered a 0.617/0.624 lift split, a 231/239-degree duration split, and a 113-degree LSA.

9 Feeding the truck intake was an 80mm Accufab throttle body.

10 Exhaust chores were handled by a set of 1-7/8-inch stainless steel headers from American Racing Headers feeding Magnaflow mufflers.

11 The motor was run with a FAST XFI/XIM management system controlling a set of Holley 75lb injectors.

12 Equipped with the stock truck intake and Accufab throttle body, Modern Mouse produced 515 hp at 5,800 rpm and 506 lb-ft at 4,700 rpm.

13 Off came the composite truck intake and on went the BBK SSI.

14 The same Holley 75lb injectors were employed on the BBK intake but run with Wilson billet fuel rails.

15 Equipped with the same Accufab throttle body, the BBK SSI intake produced 524 hp at 6,100 rpm and 503 lb-ft of torque at 4,800 rpm. The SSI intake improved power output by 15-16 hp at the top of the rev range but lost out to the truck intake below 5,500 rpm.

16 The BBK SSI intake offered 15-16 additional horsepower at the top of the rev range, but lost out below 5,500 rpm. We liked the idea of being able to port the BBK intake for improved flow. We suspect radiused port entries and port matching might really wake up the aluminum intake. We also feel the SSI intake might respond to a slightly wilder cam or larger stroker application.